(1-0001)
(1) Field of the Invention
A first aspect of the present invention relates to a sintered laminated structure including a plurality of ceramic layers made of different materials, which sintered laminated structure is useful as a filter as well as a laminated structure for electrochemical cells such as solid oxide fuel cells, steam electrolysis cells, oxygen pumps, and NOx decomposition cells. The invention also relates to a process for producing such a sintered laminated structure, and to an electrochemical cell using such a sintered laminated structure.
(2-0001)
A second aspect of the present invention relates to a self-supporting type substrate for an electrochemical cell, a process for producing the substrate for the electrochemical cell, and such an electrochemical cell, said electrochemical cell including the self-supporting type electrode substrate and adapted to be used as a solid oxide fuel cell, a steam electrolysis cell, an oxygen pump, or a NOx decomposing cell or the like.
(2) Related Art Statement
(1-0002)
Heretofore, ceramic filters have been used in various technical fields including treatment of water. Among such filters, for example, a filter in which a ceramic layer having fine pores is formed on the surface of a cylindrical porous ceramic main body is known.
(1-0003)
The solid oxide fuel cells are broadly classified into a planar type and a tubular type. In the solid oxide fuel cell of the planar type, a power generating stack is formed by alternatively laminating so-called separators and power-generating layers. In JP-A 5-54897, a fuel electrode and an air electrode are separately formed, a power-generating layer is produced by using these fuel electrode and air electrode, an interconnector is formed, and a laminated body is produced by interposing a thin film containing ceramic powder and an organic binder between the power-generating layer and the interconnector.
(1-0004)
In JP-A 6-68885, a green molded body of an interconnector and that of a distributor on a side of an air electrode are laminated together, and the interconnector and the distributor are joined together by integrally firing the resulting laminate. According to this process, a stress-relaxation layer is formed between the green molded bodies by applying such a material between them as extremely differs from the green bodies in terms of the thermal shrinkage behavior so that stress between the green molded bodies may be relaxed by the stress-relaxation layer. The above stress-relaxation layer is finely broken during firing shrinkage, which relaxes the stress.
(1-0005)
Both of a principal portion of the solid oxide fuel cell and the ceramic filter is a member obtained by laminating ceramic layers and integrally the laminate. However, they have the following common problems. That is, as to the ceramic filter, the entire dimension of the filter needs to be made greater to enhance the filtration efficiency. This causes a cost-up and increase in the installation space. Furthermore, since the filter is made of bulky ceramics, the filter has a large weight. Furthermore, in order to decrease the filtration resistance of the filter, the thickness of the filter needs to be thin. In order to decrease the filtering resistance of the filter, the thickness of the filter needs to be decreased. This unfavorably largely deteriorates the strength of the structural body of the filter.
(1-0006)
On the other hand, according to the solid oxide fuel cell, a gas cannot be fed to the electrode at good efficiency, that is, resistance against diffusion of the gas inside the electrode is large, so that there is a certain limit upon the power-generating performance of the cell. Further, there are problems that a unit cell is heavy and a material cost is high because a rare earth element is used.
(2-0002)
In the solid oxide fuel cell, it is a common technique that an electrode or an interconnector is designed in the form of a substrate to give strength, and that constituting elements (a solid electrolyte and the other electrode) other than the substrate are formed on the substrate in a filmy fashion to reduce resistance of the cell. For example, according to a solid oxide fuel cell of a so-called Westinghouse type, a tubular air electrode is used as a substrate, and a film of a solid electrolyte and a film of a fuel electrode are formed on this substrate. NGK Insulators, Ltd. also disclosed a unit cell having a structure in which a sintered laminated structure composed of an air electrode and an interconnector is used as an air electrode/interconnector substrate, and a film of a solid electrolyte and a film of a fuel electrode are formed on the substrate (JP-A-5-66518).
(2-0003)
However, since a collective cell is formed by laminating a number of planar or tubular unit cells, given mechanical strength is required for such unit cells. In the conventional unit cells, since the electrode is used as a substrate, it is indispensable that the substrate has a large thickness of around 3 mm to around 10 mm to afford given mechanical strength upon the substrate. For this reason, there exist problems in that the substrate is heavy and the cost for materials to constitute the substrate is high. Further, since the substrate is thick, an internal resistance of the substrate is large. Electric power is generated at a three-phase interfaces at which electrodes, pores and a solid electrolyte contact together. Since the substrate is thick, diffusing resistance at which a gas diffuses in the substrate is large, and polarization is great. Consequently, this leads to reduction in the electric power-generating performance. Furthermore, it has been clarified that similar problems exist in the case of an electrochemical cell other than the solid oxide fuel cell, for example, a high temperature steam electrolysis cell.
(1-0007)